Oxygen is a commodity chemical in the industrial gas industry. It has numerous applications including waste water treatment, glass melting furnaces, and the steel industry. One of the most common methods of oxygen production is by cryogenic distillation of air. However, this technology is not competitive for small size oxygen plants (&lt;100 TPD O.sub.2). The technology of choice for this size range is adsorption. There is a need in the marketplace to produce oxygen at low capital and energy costs by adsorptive gas separation.
Adsorptive processes are extensively used in the industry to produce oxygen from air for small size oxygen plants (&lt;100 TPD O.sub.2). There are two major categories of these processes -- pressure swing adsorption processes (PSA) and vacuum swing adsorption processes (VSA). The pressure swing adsorption processes carry out the adsorption (feed) step at pressures much higher than ambient and adsorbent regeneration at pressures close to ambient. The adsorbent beds go through secondary process steps, such as pressure equalizations, depressurizations, blowdowns, and purge or various combinations of these during the cycle.
These processes tend to be energy intensive and more suitable for smaller oxygen plants producing less than 40 tons of oxygen per day and preferably less than 20 tons of oxygen per day. A subset of O.sub.2 PSA processes is a rapid pressure swing adsorption (RPSA) process. As the name implies, this process involves similar steps as a PSA process, but carries out these steps very quickly. Again, this process tends to be energy intensive and suitable for oxygen plants even smaller than O.sub.2 PSA's.
Primary reasons for high energy consumption in PSA processes are: (1) O.sub.2 recovery from these processes is low, and (2) the entire feed stream has to be compressed up to the adsorption pressure. These inefficiencies are somewhat circumvented in vacuum swing adsorption (VSA) processes. In these processes, adsorption is carried out at pressure slightly above ambient and adsorbent regeneration is carried out at sub-atmospheric levels. The adsorbent beds go through several secondary steps with the primary aim of increasing oxygen recovery and reducing adsorbent inventory per unit of product gas.
U.S. Pat. No. 4,917,710 describes a two bed O.sub.2 VSA process with a product storage vessel. Process cycle steps are: adsorption, cocurrent depressurization, simultaneous cocurrent depressurization and evacuation, evacuation, vacuum purge by product, vacuum purge by gas obtained in a cocurrent depressurization step, simultaneous pressure equalization and product repressurization, and simultaneous feed and product repressurization. Gas for product repressurization and product purge is obtained from the product storage vessel. Gas for pressure equalization is obtained from the bed on simultaneous cocurrent depressurization and evacuation step.
U.S. Pat. No. 4,781,735 describes a three bed O.sub.2 VSA process with steps: adsorption, feed to feed or dual end pressure equalization, cocurrent depressurization, evacuation, vacuum purge by gas obtained in cocurrent depressurization step, product repressurization from bed on feed step, simultaneous feed repressurization and feed to feed or dual end pressure equalization.
European patent application 0 354 259 outlines various options for a two bed O.sub.2 VSA process: adsorption, cocurrent depressurization, evacuation, pressure equalization with gas obtained in cocurrent depressurization step and feed repressurization. An option includes vacuum purge by product gas from the bed on adsorption step.
U.S. Pat. No. 5,015,271 describes an O.sub.2 VSA process with the steps: adsorption, simultaneous cocurrent depressurization and countercurrent evacuation or feed, countercurrent evacuation, simultaneous product to product pressure equalization and feed repressurization, or vacuum purge, simultaneous feed and product repressurization and feed repressurization.
U.S. Pat. No. 5,122,164 describes an O.sub.2 VSA process with the steps: adsorption, simultaneous cocurrent depressurization and countercurrent evacuation, countercurrent evacuation, vacuum purge, pressure equalization with gas from a bed undergoing cocurrent depressurization and product repressurization.
U.S. Pat. No. 5,223,004 describes an O.sub.2 VSA process with the steps: adsorption, simultaneous cocurrent depressurization and countercurrent evacuation, countercurrent evacuation, purge, repressurization with product and cocurrent depressurization gas from another bed and repressurization with product and feed.
U.S. Pat. No. 5,328,503 describes an O.sub.2 VSA process with the steps: adsorption, cocurrent depressurization, simultaneous cocurrent depressurization and evacuation, evacuation, vacuum purge by gas obtained from the cocurrent depressurization step, pressure equalization with gas obtained from the simultaneous cocurrent depressurization step, and repressurization using various combinations of feed gas, product gas and ambient air.
U.S. Pat. No. 5,429,666 describes a two bed O.sub.2 VSA process with the steps: adsorption, simultaneous cocurrent depressurization and evacuation, evacuation, vacuum purge by product gas, simultaneous pressure equalization and repressurization using cocurrent depressurization gas and feed gas/ambient air respectively, and repressurization using various combinations of feed gas, product gas and ambient air.
Despite the prior art, a need still exists for an O.sub.2 VSA process with higher oxygen recovery (i.e. lower energy costs) and lower adsorbent requirement per unit of oxygen production (i.e. lower capital costs) than the current processes. The present invention outlines a vacuum swing adsorption (VSA) process to produce oxygen from air at higher oxygen recovery and lower adsorbent requirement per unit of oxygen product than current O.sub.2 VSA processes.